What reason do we have to believe that the whole package from milk is one worth receiving? I can get heaps of diverse beneficial bacteria from plenty of other sources. What’s milk offering that’s unique or special?

I’m asking sincerely. I’ve never bothered looking into it.

To answer one of your questions directly, it's the only thing that works for young mammals, it's got to have some unique mixture in it by that fact alone.

However, we're not all babies. I think lots of other sources can be just as beneficial, but this thread was about milk.